The present invention pertains to improvements in the field of electrocoagulation printing. More particularly, the invention relates to an improved anode for use in an electrocoagulation printing method and apparatus.
In U.S. Pat. No. 4,895,629 of Jan. 23, 1990, Applicant has described a high-speed electrocoagulation printing method and apparatus in which use is made of a positive electrode in the form of a revolving cylinder having a passivated surface onto which dots of colored, coagulated colloid representative of an image are produced. These dots of colored, coagulated colloid are thereafter contacted with a substrate such as paper to cause transfer of the colored, coagulated colloid onto the substrate and thereby imprint the substrate with the image. As explained in this patent, the positive electrode is coated with a dispersion containing an olefinic substance and a metal oxide prior to electrical energization of the negative electrodes in order to weaken the adherence of the dots of coagulated colloid to the positive electrode and also to prevent an uncontrolled corrosion of the positive electrode. In addition, gas generated as a result of electrolysis upon energizing the negative electrodes is consumed by reaction with the olefinic substance so that there is no gas accumulation between the negative and positive electrodes.
The electrocoagulation printing ink which is injected into the gap defined between the positive and negative electrodes consists essentially of a liquid colloidal dispersion containing an electrolytically coagulable colloid, a dispersing medium, a soluble electrolyte and a coloring agent. Where the coloring agent used is a pigment, a dispersing agent is added for uniformly dispersing the pigment into the ink. After coagulation of the colloid, any remaining non-coagulated colloid is removed from the surface of the positive electrode, for example, by scraping the surface with a soft rubber squeegee, so as to fully uncover the colored, coagulated colloid which is thereafter transferred onto the substrate. The surface of the positive electrode is thereafter cleaned by means of a plurality of rotating brushes and a cleaning liquid to remove any residual coagulated colloid adhered to the surface of the positive electrode.
When a polychromic image is desired, the negative and positive electrodes, the positive electrode coating device, ink injector, rubber squeegee and positive electrode cleaning device are arranged to define a printing unit and several printing units each using a coloring agent of different color are disposed in tandem relation to produce several differently colored images of coagulated colloid which are transferred at respective transfer stations onto the substrate in superimposed relation to provide the desired polychromic image. Alternatively, the printing units can be arranged around a single roller adapted to bring the substrate into contact with the dots of colored, coagulated colloid produced by each printing unit, and the substrate which is in the form of a continuous web is partially wrapped around the roller and passed through the respective transfer stations for being imprinted with the differently colored images in superimposed relation.
The positive electrode which is used for electrocoagulation printing must be made of an electrolytically inert metal capable of releasing trivalent ions so that upon electrical energization of the negative electrodes, dissolution of the passive oxide film on such an electrode generates trivalent ions which then initiate coagulation of the colloid. Examples of suitable electrolytically inert metals include stainless steels, aluminium and tin.
As explained in Applicant's U.S. Pat. No. 5,750,593 of May 12, 1998, the teaching of which is incorporated herein by reference, a breakdown of passive oxide films occurs in the presence of electrolyte anions, such as Cl.sup.-, Br.sup.- and I.sup.-, there being a gradual oxygen displacement from the passive film by the halide anions and a displacement of adsorbed oxygen from the metal surface by the halide anions. The velocity of passive film breakdown, once started, increases explosively in the presence of an applied electric field. There is thus formation of a soluble metal halide at the metal surface. In other words, a local dissolution of the passive oxide film occurs at the breakdown sites, which releases metal ions into the electrolyte solution. Where a positive electrode made of stainless steel or aluminium is utilized in Applicant's electrocoagulation printing method, dissolution of the passive oxide film on such an electrode generates Fe.sup.3+ or Al.sup.3+ ions. These trivalent ions then initiate coagulation of the colloid.
Stainless steels are preferred due to their low cost and availability. These are iron alloys containing a minimum of approximately 11 wt. % chromium. This amount of chromium prevents the formation of rust in unpoluted atmospheres. Their corrosion resistance is provided by the aforesaid passive oxide film which is self-healing in a wide variety of environments.
The stainless steels hitherto used by the Applicant consisted of 12 to 20 wt. % Cr, 3 to 10 wt. % Ni, 0.5 to 2.5 wt. % Mo and 0.03 to 0.09 wt. % C, with the balance consisting of iron and unavoidable impurities. Although such alloys give satisfactory results in respect of electrocoagulation, Applicant has observed that they do not have a hardness sufficient to withstand the harsh conditions encountered during cleaning of the positive electrode, resulting in abrasion and pitting of such an electrode. It is therefore necessary to regrind the surface of the electrode after every forty hours of printing. This, of course, requires shutdown of the printing apparatus and removal of the electrode.
As it is known, many elements other than chromium are added to iron to provide specific properties or ease of fabrication. For example, nickel, nitrogen and molybdenum are added for corrosion resistance; carbon, nitrogen and titanium for strength; sulfur and selenium for machinability and nickel for formability and toughness. Applicant has observed that a stainless steel with a high carbon content adversely affects passivation. A stainless steel with a high nickel content, on the other hand, is difficult to clean so that a residual film of ink containing non-coagulated colloid is left on the surface of the positive electrode and is transferred with the colored, coagulated colloid onto the substrate during contacting same. Thus, when black, cyan, magenta and yellow coloring agents are used to provide a polychromic image, the residual films containing these coloring agents upon being transferred onto the substrate in superimposed relation create on the printed image an undesirable colored background.